Hyperthermia impairs liver mitochondrial function in vitro

The effects of temperature on the relationships among the rates of pyruvate carboxylation, O-2 uptake (J(o)), oxidative phosphorylation (J(p)), and th e free energy of ATP hydrolysis (G(p)) were studied in liver mitochondria i solated from 250-g female rats. Pyruvate carboxylation was evaluated at 37, 40, and 43 degrees C. In disrupted mitochondria, pyruvate carboxylase maxi mal reaction velocity increased from 37 to 43 degrees C with an apparent Q( 10) of 2.25. A reduction in ATP/ADP ratio decreased enzyme activity at all three temperatures. In contrast, in intact mitochondria, increasing tempera ture failed to increase pyruvate carboxylation (malate + citrate accumulati on) but did result in increased J(o) and decreased extramitochondrial G(p). J(p) was studied in respiring mitochondria at 37 and 43 degrees C at vario us fractions of state 3 respiration, elicited with a glucose + hexokinase A DP-regenerating system. The relationship between J(o) and G(p) was similar at both temperatures. However, hyperthermia (43 degrees C) reduced the J(p) /J(o) ratio, resulting in lower G(p) for a given J(p). Fluorescent measurem ents of membrane phospholipid polarization revealed a transition in membran e order between 40 and 43 degrees C, a finding consistent with increased me mbrane proton conductance. It is concluded that hyperthermia augments nonsp ecific proton leaking across the inner mitochondrial membrane, and the resu ltant degraded energy state offsets temperature stimulation of pyruvate car boxylase. As a consequence, at high temperatures approaching 43 degrees C, the pyruvate carboxylation rate of intact liver mitochondria may fail to ex hibit a Q(10) effect.